1. Field of the Invention
The present invention is directed to probe-based instruments such as an atomic force microscope and, more particularly, to probe assemblies for such instruments and corresponding methods to optimize tuning of the instrument.
2. Description of Related Art
Probe-based instruments monitor the interaction between a probe and a sample to obtain information concerning one or more characteristics of the sample. Scanning probe microscopes (SPMs) typically characterize the surface of the sample down to atomic dimensions by monitoring the interaction between the sample and a tip supported by a corresponding cantilever of the probe assembly by providing relative scanning movement between the tip and the sample. As a result, surface characteristic data can be acquired over a particular region of a sample, and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM in both commercial and research environments. The probe assembly of a typical AFM includes a micron-sized cantilever supported at its base which may define, for example, a silicon cantilever extending from a silicon die section. The cantilever typically has a sharp probe tip attached to the free end opposite the base for interacting with the sample surface. In operation, the probe tip is brought very near or into contact with the surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with a highly sensitive deflection detector, most preferably an optical lever system employing a laser and a quadrant photodetector. Other deflection detection schemes include strain gauge arrangements, an arrangement of capacitance sensors, etc.
In operation, the probe assembly, and more particularly, the tip of the cantilever scans the sample surface using a high resolution three-axis scanner (e.g., a piezoelectric actuator) acting on either the sample support or the probe assembly. As a result, the instrument is capable of creating relative motion between the probe and the sample while measuring a surface characteristic such as topography or some other property of the sample.
Notably, AFMs may be designed to operate in a variety of modes, including contact mode and oscillating modes. With specific reference to oscillating modes, one preferred mode of operation causes the tip to tap on the sample surface. In this mode of operation, the tip is oscillated at or near a resonant frequency of the cantilever of the probe using a transducer such as a piezoelectric actuator. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals which are generated in response to the tip-sample interaction. The feedback signals are collected, stored and used as data to characterize the sample, for instance, to generate a map of the surface of the sample.
In this regard, when operating in an oscillating mode, ideal operation can only be realized when the cantilever is caused to oscillate at resonance. Therefore, the response of the cantilever over a range of frequencies is typically measured to determine at which frequency a maximum amplitude signal is achieved. One typical problem when conducting this preliminary operation, called tuning, is that the true mechanical resonance of the cantilever may be difficult to determine due to unwanted vibrations in the system. Such vibrations can cause mechanical resonance curves that contain peaks at frequencies near the resonant frequency of the probe, or harmonics thereof, thus making it difficult to distinguish the true mechanical resonance of the probe.
AFM users know that it is common to observe unexpected resonance spectra, such as bumps or dips, close to or even just at resonance. If these imperfections coincide with the desired frequency range that the cantilever is to vibrate, they will interfere with Imaging and result in degraded images, as well as degraded tip life. On the other hand, if such imperfections are out of the frequency range of interest, they will not interfere with the measurements.
Most often, the probe itself is not to blame for non-ideal resonance curves. AFM probes are micromechanical devices, typically built on single crystal silicon using known semiconductor microfabrication techniques. Well-known silicon fabrication technology enables accurate control over both geometrical and mechanical properties of probe assemblies. Probes having reliable mechanical characteristics and which are very stable can be achieved with these techniques. When isolated, the resonance spectra of such cantilevers are clear and free of unwanted vibrations. As shown in FIGS. 1 and 2, a probe assembly 10 includes a base or die section 12 which supports a probe 14 preferably fabricated from the same material, for example, a silicon wafer. Base 12 includes a first surface 16 and a second, generally opposed bottom surface 18 each of which is substantially flat, continuous and planar. Probe 14 includes a cantilever 20 and has a free end 22 that oscillates a tip 24 which is supported at free end 22 and includes an apex 26 that interacts with the sample under test. In operation, energy from an oscillating transducer (not shown) is coupled through the base 12 of probe assembly 10 to cantilever 20 of probe 14 to cause cantilever 20, and thus tip 24, to oscillate. The transducer is driven by an oscillating source having a frequency selected to cause cantilever 20 to oscillate at its resonance. As noted previously, the resonant frequency is an inherent property of the corresponding probe 14. Notably, it is the imperfect coupling of the energy to the probe 14 that can produce less than ideal performance of the scanning probe microscope.
Because mechanical properties of probe assemblies can be accurately controlled using known techniques, the probe assemblies themselves are typically not the source of unwanted vibrations in the system. Most often, poor mechanical resonance curves produced during tuning are caused by imperfect mounting of the cantilever. More particularly, this problem can often be isolated as an imperfect mechanical interface between the probe assembly and the cantilever holder, which compromises coupling of the oscillating energy produced by the oscillating transducer to the probe tip.
FIG. 3 shows an example of a double resonance peak in the response of a cantilever as frequency is swept over a range of about one kilohertz. More particularly, the amplitude response shown has two peaks, A and B, near the resonance frequency (308.80 kHz) of the probe of the AFM being tuned. Note that the phase response is also illustrated in FIG. 3, and although providing information regarding the imperfect tune, the amplitude response provides a more stark illustration of the problem.
As noted above, the double resonance peak shown is most often caused by the way the cantilever is mounted in the cantilever holder, rather than by the cantilever itself. An illustration of an AFM probe assembly 30 mounted in a probe holder 32 is illustrated in FIG. 4. Particularly, probe holder 32 may be a spring loaded clip having two generally opposed portions 34, 36 separated at least at one end by a support 38. Portions 34, 36 are biased against one another in this arrangement by a spring element 40 having a spring constant sufficient to secure probe 30 within holder 32 during AFM operation. Preferably, a bottom or inside surface 42 of portion 34 supports a transducer such as a piezoelectric actuator 44 that may be actuated via electrical leads (not shown) to, for instance, cause probe 46, of probe assembly 30 to vibrate. The energy generated by piezoelectric actuator 44 is coupled to probe 46 and specifically tip 48 supported by a cantilever 50 of probe 46, to oscillate tip 48 ideally at a resonance associated with probe 46. Preferably, piezoelectric actuator 44 is bonded to surface 42 of portion 34 using glue or a weld such that a surface 52 of actuator 44 defines an interface “I” between actuator 44 of probe holder 32 and probe assembly 30.
As to the cause of the response shown in FIG. 3 and described above, often times particulate matter 60 may be lodged in one of the mechanical interfaces, for example, between piezoelectric actuator 44 causing the oscillation and a base or die 54 of probe assembly 30, as shown in FIG. 4. Such matter 60 sandwiched in mechanical interface “I” causes, in this case, only a small region of contact “X” between actuator 44 and base 54 of probe assembly 30. As a result, coupling of the energy generated by piezoelectric actuator 44 to cantilever 50, and ultimately to tip 48, of probe assembly 30 is imperfect. Ideally, surface 52 of actuator 44 of probe holder 32 should lie flat or adjacent to as much of surface 56 of probe assembly 30 as possible (i.e., without interfering obstructions) to maintain a high integrity interface between the two, thus optimizing the energy coupled to probe 30.
In the past, when encountering an imperfect cantilever tune caused by a compromised mechanical interface (typically discovered by monitoring the operation of the cantilever while conducting a frequency sweep, i.e., when tuning the AFM for operation), the user simply adjusted (e.g., agitated) the probe assembly in the holder to try to improve the mechanical interface, for example, by dislodging any interfering particulate matter. At this point, the user would conduct another frequency sweep to determine whether a “clean” response results. If an imperfect tune persists, the user typically would then clean the probe holder 32, preferably with a swab of alcohol. If after performing these steps a clean profile is still not obtained, the user typically would replace the cantilever holder 32. Notably, the cause of an imperfect tune in this case often is due to the user employing a cantilever having a die thicker than 300 microns, or using a holder (e.g., a spring clip) that is stressed, for example, after repeated uses. Alternatively, the cause may be that the solder around the screw in the cantilever holder is cracked or compromised in some way.
The problems associated with such known methods of obtaining clean mechanical resonance curves are numerous. For instance, having to conduct a tune and analyze the output, and then having to remove and adjust the probe assembly with respect to the probe holder is time consuming. Such adjustments are annoying and often times do not resolve the problem and, in the end, interfere with the operator's use of the AFM. Moreover, if the adjustment of the probe assembly in the probe holder does not produce the desired result, the user must take further measures to resolve the problem, such as cleaning the probe holder. This leads to additional handling of the relatively delicate probe assembly, and again further delays use of the AFM and yet still does not guarantee that the AFM will be ready to use. And, if such known methods do not resolve the problem, the probe holder may have to be replaced entirely, clearly leading to more expense and a further overall delay in operating the SPM.
Alternatively, to minimize the risk of a compromised interface between the probe assembly and the probe holder, the user may glue the probe assembly to the probe holder, and more particularly the piezoelectric actuator to the probe holder. Although an effective resolution to achieving a solid mechanical interface between the probe assembly and the probe holder, and thus a clean mechanical resonance curve, the most significant problem associated with gluing the probe assembly to the probe holder is that often times during operation, the probe assembly must be replaced. For instance, this may occur because a tip of the probe assembly becomes damaged, or a cantilever having an alternate resonance frequency would be preferable based on the type of application. To remove a glued probe assembly from a probe holder, a solution, such as acetone, is required to break the glue bond. The problems associated with gluing the probe assembly are numerous. The process is laborious and time-consuming and requires further handling of the delicate components.
As a result, the field of AFM operation was in need of a tuning solution that makes obtaining clean mechanical resonance curves efficient. A method and/or apparatus is desired that minimizes the amount of the user's time to prepare the AFM for operation, yet maintains flexibility in terms of exchange of probe assemblies.